The present invention relates generally to air/oxygen cells, particularly button shaped cells comprised of an inner metal cup which receives the electrolyte and the negative electrode material and an outer metal cup which contains a multilayer positive air/oxygen electrode, joined with the interposition of an insulating seal.
The air electrode of such cells is multipurpose in function, its primary task being the depolarization of air/oxygen, and additional tasks including the limitation of material exchange (CO.sub.2 absorption, water exchange), especially with the surrounding atmosphere; the prevention of electrolyte escape from the portion of the housing which is accessible to the air; the sealing against the housing cup with simultaneous good electrical contact; and the absorption of closure forces developed during the crimping process.
Regarding construction, such air electrodes are generally composed of an active carbon layer provided with a mesh conductor which serves as the actual depolarizing layer, and a hydrophobic cover layer (usually an unsintered PTFE foil) located on the air side of the electrode. If appropriate, the active carbon can be provided with a catalyst. On the air side, there is further generally provided a porous diffusion layer to improve air distribution.
In the assembly of such button cells, the introduction of the air electrode can create difficulties because it consists of a very thin multilayer structure which must be retained precisely within the cell in order to perform its complicated function in trouble-free manner. However, in crimping housing halves which have previously been provided with such cell components, it can happen that the air cathode does not remain in level position but becomes bowed as a result of displacement or plastic deformation during the crimping process. This can result in excessive material exchange with the atmosphere, escape of electrolyte through cracks in the hydrophobic cover layer, loss of contact between the mesh conductor and the cathode cup, separation of the depolarizing mass from the mesh conductor, or separation of the cover layer resulting from increased electrolyte pressure occurring during discharge.
In an effort to overcome such difficulties, a button cell described in German Pat. No. 2,454,890 achieves secure, electrolyte-fast retention of the air cathode by having the bottom of the cell cup and of the cathode bowed convexly toward the outside, and by holding the rim of the cathode firmly between the sealing base and a conforming step-shaped rim region of the cell cup.